1. Field of the Invention
The present invention relates generally to an idle speed control method and system for an internal combustion engine of an automotive vehicle. More particularly, the present invention relates to a control method and system for controlling idle speed by controlling an intake air flow rate, including correcting a control value which corresponds to the duty cycle of a pulse signal to be applied to a mechanical air flow rate control means electrically operative in response to the control value is thereby limited to prevent its entering into the deadband of the mechanical means.
2. Description of the Prior Art
In recent years, pollution of the atmosphere by nitrogen oxides NO.sub.x, carbon monoxide CO, gaseous sulfurous acid and so on produced in the exhaust gas of automotive vehicles has become a serious problem. In addition the price of automotive fuel, i.e. gasoline or petrol, has increased. To prevent atmospheric pollution caused by automotive exhaust gas and to promote fuel economy, it has become necessary to accurately control engine speed even when the vehicle engine is idling.
In order to control idle speed by controlling the air flow rate, it is known to provide in the air intake passage an electrically operative mechanical air flow rate control means, such as electromagnetic valve means. Generally speaking, such mechanical means operates in response to application of a pulse signal indicative of a pulse duty cycle. The pulse duty cycle, used to determine the ratio of energizing period and deenergizing period of the mechanical means, is defined as the pulse ratio in one cycle of pulse signal to be input to the mechanical means. Depending on the pulse width of the pulse signal, the control value is determined by the duty cycle to control opening and closing of the valve means. The mechanical air flow rate control means includes dead bands or zones wherein the operating characteristics thereof, responsive to varying of the pulse duty cycle are significantly lessened. When the control ratio enters the dead band range of the mechanical means, a response delay occurs. For example, as shown in FIG. 3, control signal S.sub.3 is determined by the sum of an open loop control signal S.sub.1 and a closed loop control signal S.sub. 2. The open loop control signal S.sub.1 corresponds to the engine or coolant temperature and the closed loop control signal S.sub.2 corresponds to the difference between the actual engine speed and a reference engine speed determined as a function of coolant temperature. In response, to increasing of engine speed and increasing of the engine temperature, the control signals S.sub.1, S.sub.2 of both the open loop and the closed loop controls are decreased gradually to enter into the dead band of the mechanical means which is either above a maximum rate K.sub.H or below a minimum value K.sub.L.
In the conventional system, upon starting the engine at time T.sub.1, the air flow rate is controlled by feedback control within a period of time W.sub.1, and is increased corresponding to the required rate. Thereafter the pulse signal duty cycle, represented by the control value is gradually reduced to the normal control value. However, at this time, if the vehicle is driven at point T.sub.2 so that open loop control is carried out, the feedback signal S.sub.2 is fixed at its value immediately before starting the vehicle. Since, at this time, the engine speed is gradually decreased from the initial value by feedback control, the feedback control signal S.sub.2 is negative during the period W.sub.1 and therefore the fixed closed loop control signal S.sub.2 is also negative after time T.sub.2. On the other hand, according to increasing engine temperature, the open loop control signal S.sub.1 is decreased after T.sub.2. However, in the open loop control, the control value is not decreased to a value less than zero as represented by S.sub.3 ' in FIG. 3. The control signal S.sub.3 is thus fixed at zero. Accordingly, the control value of S.sub.3 enters into the dead band S.sub.4 of the mechanical means, so that a delay in response results. If, at point T.sub.3, after driving the vehicle for a period of time W.sub.2, the engine returns to idling, then the control operation is switched to closed loop control. At this time, the feedback control signal S.sub.2 is maintained at the previously fixed value which is less than zero. In response to the switching of the control operation and the lack of the air flow rate, the closed loop control value of S.sub.3 will increase rapidly to follow the change of required air flow rate. However, at this time, with the control value of S.sub.3 being less than the minimum value K.sub.L of the dead band of the mechanical valve means, the response characteristic of the mechanical valve means is quite low for a time period .tau., thereby failing to permit sufficient increase of the air flow rate. As a result, the engine may possibly stall.
To prevent such a possibility of delay of response, and to improve response characteristics of the mechanical means, it will be required to limit the range of control values so that the mechanical valve means can respond to variation of the control value without substantial delay. In the present invention, therefore, the control value is limited to be within a range of 10 to 80 percent of the maximum control value assuming a value of 100 percent to represent one cycle of pulse signal.